Yeah, I've found a pretty good one, but none of the really good ones are free.
Its how I'm going to connect my Digital Piano up. I'm going to use the computer as an interface to get to the better sound. (My digital piano sound has a lot to be desired)
Try out a demo of Moddart's PianoTeq for an example of what can be done nowadays. The difference between this one, and the rest, is this actually uses your CPU to create the piano sound, and no samples are used, so its a very small download.
Other alternatives (also not free, but sample based), are NI Acoustik Piano, Ivory, and Apsolute software's.
The sample ones are at the moment better than the data driven based piano's, but they are getting better, and can produce very accurate results, since they have the power of each note sounding very different to each other, and a wide range of effects being put in place that might not be possible with a sample based range.
A VST though called the MDA Piano is the best freeware alternative:
It'll probably need a piece of software though to run VST's. A VST is a plugin:
"Virtual Studio Technology. The audio engine created by Steinberg for their Cubase program"
But almost if not all audio software support VST's now.
A nice freeware audio creation software tool, lies here:
Renoise. Easily one of the best trackers available, which with effort could create professional tracks. The interface needs getting used to, but that'll read VTS's no problem.